This invention relates generally to optical coatings for controlling the reflection and transmission of particular optical wavelengths at an optical surface. This invention relates particularly to optical coatings for ring laser gyroscope mirrors. Still more particularly, this invention relates to optical coatings that provide resistance to degradation of mirror surfaces under exposure to ultraviolet wavelengths in a ring laser gyroscope.
A ring laser gyroscope employs the Sagnac effect to measure rotation. Counterpropagating light beams in a closed path have transit times that differ in direct proportion to the rotation rate about an axis perpendicular to the plane of the path. In a ring laser gyroscope the closed path is defined by mirrors that direct the light beams around the path. The path is generally either square or triangular in shape, although any closed polygonal path could, in principle, be used. The closed path is typically in a cavity formed in a frame or body that is formed of a glass ceramic material.
The cavity is evacuated and then filled with a mixture of helium and neon, which is the gain medium for the laser. An electrical discharge excites the gain medium, which produces light amplification and a coherent light source. The mirrors must be precisely aligned to direct the light beams around the closed path in a manner that promotes proper operation of the laser. The mirror surface must be free of impurities to provide a laser beam intensity that will result in a usable signal.
Once laser oscillation occurs in the system at resonant frequencies, the difference in the length of the pathways traversed by the counterpropagating laser beams results in a difference or beat frequency which is sensed by a photodetector and amplified by an amplifier. The beat frequency is a result of optically heterodyning the counter propagating beams.
In almost all lasers there is a decrease in useful laser power output over the operating life of the laser. This decrease in useful power is accentuated by losses at optical elements. With lasers that lase at relatively low gain a decrease in useful power can become very significant. With lasers that operate at high gain (such as lasers which lase at some visible light frequencies) losses from optical elements, while undesirable, can often be tolerated during the operating life of the tube. However, even with such high gain lasers, eliminating or minimizing such losses is desirable. Eliminating or minimizing optical element losses can extend the useful life of the tube and provide more efficient and precise operation of the tube during its useful life.
The plasma arc generated in the tubes of gas ion lasers can produce large photon fluxes which are capable of initiating physical and chemical changes on an optical element surface exposed to the fluxes. More particularly, these photon fluxes are capable of producing photo reduction of the exposed optical element surfaces.
Exemplary materials that have been used for optical elements in such lasers includes but is not limited to crystalline SiO.sub.2, Si, fused SiO.sub.2, sapphire, diamond, BeO, MgF.sub.2, ZnS, ZnSe, BaF.sub.2, CaF.sub.2, diamond-like carbon, yttrium aluminum garnet (YAG), yttrium lithium fluoride (YLF), and the like. In particular, ring laser gyroscope mirrors generally include multiple alternating layers of SiO.sub.2 and TiO.sub.2 arranged in a stack. These materials often experience physical and chemical changes, particularly photo reduction, on the surface exposed to the photon flux. The ZLG mirrors made with a SiO.sub.2 /TiO.sub.2 multilayer stack show a UV/plasma degradation in terms of output power drop and mirror birefringence shift.
Dielectric coatings for optical applications are generally formed by vacuum evaporation, sputtering or low temperature solution deposition over a suitable glass, ceramic or metal substrate. U.S. Pat. Re. No. 32,849 issued Jan. 31, 1989 to Wei et al., U.S. Pat. No. 4,827,870, issued May 9, 1989 to Lee and U.S. Pat. No. 4,793,980, issued Dec. 27, 1988 to Scott et al. disclose apparatus and methods that may be used to form dielectric coatings on mirror substrates. The disclosures of U.S. Pat. Re. Nos. 32,849, 4,827,870 and 4,793,980 are hereby incorporated by reference into the present disclosure.
The particular optical function and the wavelength or wavelengths of use for the optical coating dictates the coating design. Here the term coatings design refers to the number of discrete layers of material to be deposited, the thickness of these layers and materials from which the layers are to be fabricated. The difference in refractive index between the materials that form the discrete layers is the physical property that, in combination with the coating design, gives the coating its unique function. For example, coatings can be designed to function as reflectors, antireflectors, polarizers and other optical elements.
It would be an advancement in the art to provide laser optical components which would not undergo photo reduction when exposed to large photon fluxes, particularly when the laser is one which generates ultraviolet radiation during operation. Such lasers produce ultraviolet radiation either incidental to or as a part of the beam and include noble gas ion lasers, excimer lasers, CO.sub.2 lasers, free electron lasers, atomic metal vapor lasers, and the like.